The present invention relates to apparatus and methods for modulating refractive index. More particularly, the invention relates to inducing a change of refractive index in one or more layers of an iono-refractive structure that are capable of ion intercalation or release. The ion intercalation or release is accompanied by a change of refractive index without a significant increase in the optical absorption at the wavelength of interest or at the operating wavelength. The structure may be suitable for the fabrication of or integration with tunable optical filters, wavelength-selective optical elements, active modulated interferometers, variable phase shifters, sensors, lenses, and other optical components.
Various approaches have been employed for modulating refractive index in a wide range of optical devices including interferometric optical waveguide-based switches, light detection and ranging (LIDAR) remote sensors, optical Doppler Tomography, filters for optical communications, and instruments to detect displacement. In most instances, modulating refractive index involves application of a voltage across a liquid crystal film, or across an electro-optical film, such as lithium niobate, or through carrier induced refractive index changes in, for example, gallium arsenide (GaAs). Elasto-optic, magneto-optic, and acousto-optic effects can also provide a change in refractive index.
It has been shown that the refractive index of many semiconducting metal oxides depends on the degree of intercalation of ions entering the matrix—i.e., the extent to which ions are inserted into the metal oxide. For example, the real and imaginary portions of the refractive index of tungsten oxide at 550 nm change significantly as lithium ions are inserted into the tungsten oxide (Rubin et al., “Optical Indices of Lithiated Electrochromic Oxides,” in Lawrence Berkeley National Labs Publication 39410 (1996)). As described by Rubin, the real portion of the refractive index changes from 1.95 to 1.7 after ion intercalation while the imaginary portion of the refractive index changes by 0.1. At longer wavelengths, there may be little or no change of the real portion of the refractive index as the imaginary portion of the refractive index changes from an extinction coefficient of 0 to 0.65.
Meanwhile, a considerable change in the refractive index of sputtered tungsten oxide with lithium ion insertion has been observed in the 2 micron to 12 micron wavelength range (Hutchins et al., “Infrared Reflectance Modulation in Tungsten Oxide Based Electrochromic Devices,” Electrochemica Acta, Vol. 46, at 1983–1988 (2001)). Such a change in refractive index of tungsten oxide could be used to create a front surface reflectance device (Hutchins et al., “Electrochromic Tungsten Oxide Films for Variable Reflectance Devices,” Proceedings of the SPIE, Vol. 4458, at 138–145 (2001)).
This follows in part from the relationship between the real portion of the dielectric constant, ∈r, determined by the phase velocity, and the imaginary portion of the dielectric constant, ∈i, determined by the absorption. The dielectric constant is also proportional to the square of the refractive index, thus the refractive index also has a real portion and an imaginary portion. It is well known from the Kramers-Kronig equation that the real portion of the dielectric constant can be expressed as an integral of the imaginary portion.
                                          ɛ            r                    ⁡                      (                          ω              0                        )                          =                ⁢                  1          +                                                    2                π                            ·              P                        ⁢                                          ∫                0                                  +                  ∞                                            ⁢                                                                    ω                    ·                                                                  ɛ                        i                                            ⁡                                              (                        ω                        )                                                                                                                        ω                      2                                        -                                          ω                      0                      2                                                                      ⁢                                                                  ⁢                                  ⅆ                  ω                                                                                                                  ɛ            i                    ⁡                      (                          ω              0                        )                          =                ⁢                                            -                              2                π                                      ·            P                    ⁢                                    ∫              0                              +                ∞                                      ⁢                                                                                ω                    0                                    ·                                      (                                                                                            ɛ                          i                                                ⁡                                                  (                          ω                          )                                                                    -                      1                                        )                                                                                        ω                    2                                    -                                      ω                    0                    2                                                              ⁢                                                          ⁢                              ⅆ                ω                                                                          where ω is the complex angular frequency, ωo refers to the frequency of an optical transition, and P is the principal value of the integral. It should be noted that the Kramers-Kronig equation may not be completely valid for thin films (Leveque et al., “Ellipsometry on Sputter-Deposited Tin-Oxide Films: Optical Constants Versus Stoichiometry, Hydrogen Content, and Amount of Electrochemically Intercalated Lithium,”Applied Optics, Vol. 37, at 7334–7341 (1990)).
However, many of these approaches that provide a change in refractive index are based on the alteration of the extinction coefficient, density, or coloration of their respective devices. Such approaches are described, for example, in Lach et al. U.S. Pat. No. 6,498,358. Moreover, while each of these approaches provides some change in refractive index, these approaches generally suffer from high manufacturing costs, constraints on the operating environment (e.g., temperature and pressure), and drawbacks related to system and substrate compatibility. Even further, many of these approaches significantly change the transmissivity while changing the refractive index, thereby limiting the applicability of the device.
It would therefore be desirable to provide apparatus and methods for modulating refractive index without significantly altering the transmissivity.
It would also be desirable to provide apparatus and methods for modulating refractive index that is easily integrated with optical components.
It would also be desirable to provide apparatus and methods for modulating refractive index via the insertion of an ionic species into an intercalating metal oxide without significantly altering the transmissivity of the metal oxide.